1. Field of the Invention
The present invention relates to an optical compensation plate which is used for a liquid crystal display and so on to compensate a phase difference of light.
2. Description Related to the Prior Art
A liquid crystal projector, in which an image and so on displayed on a liquid crystal display (LCD) is magnified and projected toward a screen, is widespread. As is generally known, the liquid crystal display has a structure that polarizing plates are disposed in a cross Nicol arrangement on both sides of a liquid crystal panel, and displays an image and so on by controlling an orientation state of liquid crystal molecules of every pixel to regulate light transmission. In addition, as for the liquid crystal panel, since a thickness and so on of a liquid crystal layer is regulated to precisely display white and black with perpendicularly incident light, it occurs a component to leak out from the emitting side polarizing plate even when displaying black, in case there is light passing the liquid crystal layer diagonally.
In the liquid crystal projector, since contrast of the projection image decreases when a leak of light occurs by the oblique incidence, a phase difference of diagonally incident light is compensated by an optical compensation plate to improve contrast. As the optical compensation plate, for example, one in which a phase difference compensation layer is formed by a dielectric multilayer film layer where two kinds of dielectric thin films varied in an index of refraction are alternately laminated on a glass substrate (so-called negative C-plate), and one in which a phase difference compensation layer is formed by an oblique deposition film made by depositing an inorganic material from a diagonal direction (so-called O-plate) are known by United States Patent Application Publication 2010/0026918 (corresponding to International Patent Application Publication 2008/078764).
Usually, an antireflection layer is established to suppress a surface reflection on a surface of an optical element. The optical compensation plate is no exception, and it is preferable that the antireflection layer is established on its surface. As the antireflection layer, for example, it is used a dielectric multilayer film layer consisting of a dielectric multilayer film where dielectric thin films varied in an index of refraction are alternately laminated. In this way, a malfunction hardly occurs in case the antireflection layer formed by the dielectric multilayer film layer is additionally formed on the optical compensation plate where the phase difference compensation layer is formed by the dielectric multilayer film layer. In addition, in this case, the phase difference compensation layer itself may have antireflection characteristics.
On the other hand, as for an optical compensation plate in which a phase difference compensation layer is formed of an oblique deposition film, there is a problem that the optical compensation plate is charged with electricity and becomes easy to adsorb dust and dirt, in case a dielectric multilayer film layer is formed on the oblique deposition film. When dust and dirt attach to the surface of the optical compensation plate, rightly quality of a display image decreases. Even if there is translucency in extraneous dust and dirt, since a phase difference is not compensated precisely in the part of extraneous dust and dirt, contrast performance turns worse. Especially in case of the optical compensation plate is used for the liquid crystal projector, since an image is magnified and projected, projection image deterioration becomes easy to be outstanding even by minute dust and dirt.